Speaking as someone who's backed a few Kickstarters, including a game (Artizens):

Early access should be built-in to any pledge level that gets you the game.

As a backer, I want to feel that my voice is heard during the development process; polls asking about what features people want (and acknowledging the results) and prompt developer responses to comments are a good way to address this.

Also, a modest discount from the launch price is a very good incentive for people like me - it can make the difference between "eh, I might get it if it launches and the reviews are good" and "ooh, I want in on this".